Generally, the combined cycle power plant includes gas turbines, steam turbines and the heat recovery steam generator (HRSG) systems. The HRSG systems are used for recovering heat in exhaust gases from the gas turbine and then using the recovered heat to produce and supply steam to the steam turbine. In general, the combined cycle power plant includes various kinds of water reservoirs such as boiler drum, deaerator reservoir and condenser reservoir. Normally, the boiler drum provides a stable water supply to the HRSG system and accumulates heat energy for compensating for changes in the amount of steam generated or consumed in the power plant. The boiler drum water level control may be significant for both plant protection and equipment safety, and can apply to high and low levels of water within the boiler drum. Additionally, a water vapor separator is especially provided inside the boiler drum so that wet steam may not be introduced into the steam turbine. Maintaining the boiler drum water level within limits is significant, as exceedance will often lead to trips or damage the equipment used in the HRSG system.
Conventional approaches for controlling the level of steam in the steam drum rely upon detecting and reacting to changes in drum level, and the balance of mass flow into, and out of, the boiler. However, these approaches are ineffective at controlling parameters for boiler swell and water level trips, causing the boiler to react too slowly to changes in demand at the steam turbine.